Java isPrime (long) Example Code

Hello, I am posting a relatively simple method to check if a number is primer or not (provided it fits in a positive java long variable). 

It is not perfect. 

It could fail. 

It should compile in all java versions AFAIK.

package gigasblade;

// If you have any comments and/or suggestions, please let me know!
public class IsPrime {
    private static boolean isPrime(final long n) {
        if (n <= 0) {
            throw new IllegalArgumentException();
        }
        if (n <= 3) {
            return true;
        }
        final long sqrtNPlus1 = (int) Math.sqrt((double) n) + 1;
        for (int i = 2; i <= sqrtNPlus1 ; i++) {
            if (n % i == 0) {
                return false;
            }
        }
        return true;
    }

    private static void checkIsPrime(long n) {
        try {
            System.out.println("isPrime(" + n + "): " + isPrime(n));
        } catch (IllegalArgumentException e) {
            System.err.println("isPrime(" + n + "): " + e);
        }
    }

    public static void main(String[] args) {
        for (int n = -5 ; n <= 18; n++) {
            checkIsPrime(n);
        }
    }
}

isPrime(-5): java.lang.IllegalArgumentException
isPrime(-4): java.lang.IllegalArgumentException
isPrime(-3): java.lang.IllegalArgumentException
isPrime(-2): java.lang.IllegalArgumentException
isPrime(-1): java.lang.IllegalArgumentException
isPrime(0): java.lang.IllegalArgumentException
isPrime(1): true
isPrime(2): true
isPrime(3): true
isPrime(4): false
isPrime(5): true
isPrime(6): false
isPrime(7): true
isPrime(8): false
isPrime(9): false
isPrime(10): false
isPrime(11): true
isPrime(12): false
isPrime(13): true
isPrime(14): false
isPrime(15): false
isPrime(16): false
isPrime(17): true
isPrime(18): false

Process finished with exit code 0